Saccades to moving targets

The metrics and dynamics of saccades to stationary and moving targets were observed in monkeys (Macaca mulatta). To isolate the effects of target speed on the saccade from contributions of smooth pursuit, saccade velocity was corrected for intrasaccadic pursuit velocity on a trial-by-trial basis prior to analysis. The effects of presaccadic retinal error and target speed on the saccadic velocity profile were determined by analyzing the partial correlations computed as a function of time after saccade onset. The main results are: (1) Saccade amplitude is determined not only by the retinal error sampled before the saccade, but also by the speed of the target during the latency period. (2) The dynamics of saccades, even if compensated for smooth-pursuit components, differ between forward- and backward-moving targets. (3) Whereas the presaccadic retinal error affects the eye velocity throughout the saccade, target speed has no effect before peak velocity. These results are discussed in the context of current models of saccade generation and their physiological substrates, in particular the role of the cerebellum in the local feedback loop.